Electronic circuits are commonly provided on and within a printed circuit board (PCB), which is covered by a protective molded compound or mold cap. The combined PCB and mold cap may be divided, or singulated, into separate solid state circuit packages, which may be covered with external shields to form discrete shielded packages, referred to as “modules” or “shielded modules.” The modules include electronic components for various electronic devices. The external shields typically comprise layers of conductive material, such as copper or other metal, that cover the tops and sidewalls of the modules (while the bottoms of the modules are protected by the PCB substrate). The external shields protect the electronic components within the modules from electromagnetic interference (EMI) caused by externally generated electromagnetic radiation, and from environmental stresses, such as temperature, humidity (e.g., hermetic sealing), and physical impact, for example.
Notably, decreasing distances between electronic components within each module tend to lead to EMI among these electronic components, caused by internally generated electromagnetic radiation, causing performance degradation. The external shield provides no shielding of the electronic components from internally generated electromagnetic radiation, including capacitive and inductive coupling and other cross-talk, for example. Indeed, the external shield, in some cases, may aggravate the electromagnetic interference by reflecting the internal electromagnetic radiation back toward the electronic components within the module. Therefore, internal EMI shields may be formed around all or part of an electronic component to provide protection from the internally generated electromagnetic radiation. An internal EMI shield that surrounds (or partially surrounds) one or more electronic components within a module may be referred to as a “compartment EMI shield.” A compartment EMI shield typically extends from a top surface of the PCB to a bottom surface of the external EMI shield, effectively creating a wall of shielding material internal to the module, protecting the one or more electronic components within the compartment formed (at least in part) by the compartment shield.
Using conventional techniques, compartment EMI shields, as well as other internal EMI shields configured to provide EMI barriers among electronic components within a module, are difficult and time consuming to implement, which tends to increase fabrication cost. For example, EMI shields may be formed using dispensing needles, which inject conductive epoxy into the top of the mold cap, in a piecemeal manner, where the epoxy ejections collectively form the EMI barrier. However, as mentioned above, this technique is time consuming since numerous, singular injection may be required. Also, the resulting EMI shield is likely to have some defects, such as epoxy dimples, voids and/or overflow, particularly after the epoxy is exposed to heat for curing (oven cured).
FIGS. 8A, 8B and 8C are cross-sections of mold caps showing examples of typical conductive epoxy defects in a portion of an internal EMI shield, e.g., formed using dispensing needles. FIG. 8A shows an epoxy dimple 810, which is a concave surface of the epoxy (which may be subsequently filled with the external EMI shield material), typically caused by shrinkage of the epoxy during oven curing. FIG. 8B shows an epoxy void 820, which is an opening in the middle section of the epoxy, typically caused by incomplete injection of the epoxy using the dispensing needle and/or shrinkage of the epoxy during oven curing. FIG. 8C shows epoxy overflow 830, which involves the epoxy overflowing the hole in the mold cap created by the dispensing needle, typically caused by an excess amount of epoxy being injected into the mold cap. As further shown in FIGS. 8B and 8C, the epoxy void 820 and the epoxy overflow 830 may also be accompanied by epoxy dimples 822 and 833, respectively, due to shrinkage of the epoxy that remains in the dispensing needle hole during oven curing.
Accordingly, there is a need for enhanced internal EMI shields among and between electronic components within solid state circuit packages, and improved fabrication techniques for achieving the enhanced internal EMI shields.